Thermostatic mixing valves commonly employ a valve shuttle movable between hot and cold seats to control the relative proportions of hot and cold water supplied to an outlet in accordance with user selection of the outlet water temperature and a thermal control system to adjust the position of the valve shuttle to compensate for changes in the temperature and/or pressure of one or both supplies tending to change the set temperature. The known valve shuttles typically have a very small stroke, for example movement of the valve shuttle from full cold to full hot is generally less than 1 mm and is typically only 0.6 mm. As a result, misalignment of the valve shuttle affects the flows of hot and cold water and this can have a significant effect on the operation of the valve. For example, if the valve shuttle lifts off the hot seat unevenly, more of the hot water flows through one side of the valve and vice versa more of the cold water flows through the opposite side of the valve giving rise to asymmetric streams of hot and cold water producing incomplete mixing of the streams that affects the response of the thermal control system to correct any deviation in the outlet water temperature from the selected temperature. It has been proposed to employ close fit sliding guides to keep the valve shuttle aligned with the seats but the sliding parts add complexity, increase manufacturing costs and are susceptible to corrosion and lime-scale causing friction. Misalignment of the valve shuttle may also result in vibrations of the valve shuttle generating noise, especially under high pressure operating conditions. Thus, the water velocity at the edge of the valve shuttle produces a low pressure region that tends to pull the valve shuttle towards its seat and any misalignment of the valve shuttle causes the pull to be uneven and this can start vibration of the shuttle valve against its seat in what we believe is a nutating motion generating noise.
Typically, the valve shuttle is mounted on a thermostat and the thermostat is displaced against the biasing of a return spring. Traditionally, the return spring is a helical coil spring of wire of circular cross-section and this may contribute to misalignment of the valve shuttle. In particular, the final turn of wire at either end of the spring coils around, not as desired in a plane perpendicular to the helical axis, but at an angle to the perpendicular. As a result, the valve shuttle mounted on the thermostat can be forced out of line with the valve seats by the inclination of the final turn of the helical wire at the ends of the spring causing the thermostat, and thus the valve shuttle carried by the shuttle, to be tilted slightly relative to the axial direction. This problem persists even if the best quality helical wire springs are used.
Generally, thermostatic mixing valves can correct for inlet water temperature changes much better than inlet pressure changes. If the flow rate is reduced by restricting the valve outlet, then inlet pressure changes become much more severe for the valve to correct. FIG. 6 shows a graph of pressure loss ratio versus temperature of the mixed water at the outlet of a typical thermostatic mixing valve for a set temperature of 40° C. typically chosen for showering. Pressure loss ratio is the ratio of the higher inlet pressure drop to the lower inlet pressure drop across the mixing valve. Normally, higher hot water pressure results in increases in the temperature of the mixed water at the outlet and higher cold water pressure results in decreases in the temperature of mixed water at the outlet. The temperature deviations for pressure loss ratios tending to increase the set water temperature are higher than those tending to reduce the set water temperature because the set temperature is usually closer to the hot water inlet temperature than the cold water inlet temperature. As shown the overall spread of temperature variation is about 6° C.
This is unacceptable for many applications, for example in healthcare installations, and currently the performance requirements for these applications are met by skewing the response of the valve to reduce the size of hot deviations which may give rise to a risk of scalding with a consequential increase in the size of cold deviations which although noticeable to the user present less risk. The hot and cold water streams are often incompletely mixed as they flow past the thermostat and changes to the waterway geometry can alter the temperature at the thermostat. As a result, skewing the response is usually done on a trial and error basis until a response is achieved that meets the standard. This is inefficient and there is still a possibility that a valve could be used under conditions in which the cold water and hot water pressures are not equal resulting in hotter temperature deviations than intended. Moreover, it may not always be possible to meet the performance requirements by skewing the response of the valve.